In a speech and audio encoding algorithm, due to the limitation of the bit rate and the characteristics of the hearing sense of human beings, low frequency signals are normally encoded with priority, whereas high frequency signals are sometimes discarded. With the development of communication network, the limitations of bandwidth become less severe, so that higher requirements on tone quality can be met. The tone quality of signals can be well improved by increasing signal bandwidth, that is, by restoring the high frequency signals. When no bit or only a few bits are available, a bandwidth expansion technique is used. The bandwidth expansion technique is a technique of expanding the speech and audio signal bandwidth range to improve the signal quality. In recent years, the bandwidth expansion technique has been developed quickly, and it is commercially applied in some fields. The bandwidth expansion algorithm in G.729.1 and the Spectral Band Replication (SBR) in the Moving Picture Experts Group (MPEG) are two bandwidth expansion algorithms that are extensively used in speech and audio compression presently.
The high frequency signals can be classified into transient signals and non-transient signals. A transient signal is a signal whose energy level has large instant changes in the time domain. In an existing method for encoding high frequency signals, when a high frequency signal is encoded, a transient signal and a non-transient signal can be encoded in different manners. But due to the development of the time-frequency transformation technique, the method for encoding a high frequency signal may no longer be compatible with the current time-frequency transformation technique. Therefore, the existing method for encoding a high frequency signal may degrade the performance of transient signals.
In an existing method for decoding a high frequency signal, a high frequency excitation spectrum is normalized according a calculated length of a frequency envelope. As a result, the deviation of the characteristics of the restored high frequency signal from the characteristics of the actual high frequency signal can be very large.